It has been required to improve emission control for an internal combustion engine mounted in an automotive vehicle. It is necessary for a diesel engine, which is a compression ignition type engine operated with diesel oil, to reduce smoke and exhausted diesel particulates such as SOF contained in exhaust gas, in addition to CO, HC, NOx. To this end, a diesel particulate filter (DPF) is provided in an exhaust pipe for trapping the exhausted particulates contained in the exhaust gas.
In the diesel particulate filter, the exhaust gas flowing into the filter passes through porous partitioning walls and the exhausted particulates contained in the exhaust gas are trapped by surfaces or micro holes of the partitioning walls. In the case that an amount of the exhausted particulates trapped and accumulated in the filter is excessively increased, a back pressure of the engine is increased due to an increase of flow resistance, and as a result an output of the engine may be decreased. Accordingly, it is necessary to properly remove the trapped exhausted particulates from the diesel particulate filter, to re-generate the same.
It is known in the art that an oxidation catalyst of platinum or the like is formed in the particulate filter, to make it possible to re-generate the particulate filter during an operation of the engine by use of the oxidation reaction of the oxidation catalyst. According to such a prior art, a fuel is supplied to the particulate filter, for example, by a post injection (in which the fuel is injected during an exhaust stroke of the engine). The accumulated exhausted particulates, which are relatively difficult to remove by a normal fuel injection, are removed from the particulate filter by oxidation with the combustion heat of the fuel (injected during the exhaust stroke).
A fuel consumption ratio will be decreased when an operation for re-generating the particulate filter is often performed. On the other hand, in the case that an interval for the re-generating operation is made longer, an excessive amount of the exhausted particulates may be trapped and accumulated in the particulate filter. As a result, the particulate filter may be likely to be damaged by an extremely increased high temperature due to rapid combustion of the trapped exhausted particulates during the re-generating operation.
Accordingly, it is desirable to decide an appropriate interval for the re-generating operation in consideration of the accumulated amount of the exhausted particulates. For example, in Japanese Patent Publication No. H7-332065, a pressure difference between pressures at an inlet and an outlet portion of the particulate filter is detected. The pressure difference is increased due to the increase of the flow resistance caused by the increased accumulated amount of the exhausted particulates. In the above Japanese Patent Publication, it is determined that the particulate filter is to be re-generated when the detected pressure difference exceeds a predetermined value.
It is, however, difficult in the prior art described in the above Japanese Patent Publication, to precisely detect the accumulated amount of the exhausted particulates and to determine the correct timing for the re-generating operation, since an actual accumulated amount of the exhausted particulates may differ from case to case even when an operating condition of the engine (including the pressure difference) is the same.
This is because there are two kinds of accumulated conditions of the exhausted particulates. Namely, in one of the accumulated conditions, the exhausted particulates are accumulated in the micro holes of the particulate filter, whereas the exhausted particulates are accumulated (in a form of multiple layers) on the surfaces of the particulate filter in the other type of the accumulation. And the exhausted particulates accumulated in the micro holes are more easily combusted than the exhausted particulates accumulated in the surfaces of the particulate filter, since a ratio of the exhausted particulates in the micro holes which are in contact with the oxidation catalyst is higher than that of the exhausted particulates on the surfaces of the particulate filer. A deviation of combustion amount may occur in a part of the particulate filter, or combustion may be discontinued, as a result of deviations of exhaust gas temperatures or exhaust gas flows. Furthermore, an inclination of a characteristic curve for the accumulation in the micro holes is much higher than that for the accumulation on the surfaces, and thereby a change of the pressure loss becomes much larger in the case in which the amount of the accumulated exhausted particulates is changed in line with the characteristic curve for the accumulation in the micro holes, than the case in which the amount of the accumulated exhausted particulates is changed in line with the characteristic curve for the accumulation on the surfaces. Accordingly, it is difficult to precisely detect the accumulated amount of the exhausted particulates.